The present invention relates to columns in which mass transfer and/or heat transfer between fluid streams occurs and, more particularly, to cross-flow trays used in such columns and methods of effecting mass transfer and/or heat transfer using such trays.
Various types of cross-flow trays have been developed to facilitate contact and thus increase the mass transfer and/or heat exchange between fluid streams flowing within processing columns. A plurality of such trays are normally used within the open internal region of such columns, with each tray extending horizontally across the cross section of the column and adjacent trays being vertically spaced apart. The trays include a deck surface in which a plurality of openings are provided to allow an ascending fluid stream, typically a vapor stream, to pass through the tray deck and interact with a liquid stream flowing horizontally across the deck surface. The deck surface that contains these openings is generally referred to as an active area. The vapor-liquid interaction that occurs on this active area of the deck surface forms a froth that facilitates the desired mass transfer and/or heat exchange between the liquid and vapor streams.
These vapor-liquid cross-flow trays also typically include larger openings and associated structures known as downcomers that allow the liquid stream to be removed from the deck surface on one tray and directed to an inlet area on the deck surface of an underlying tray. The inlet area is normally imperforate to prevent liquid from descending through the deck surface without first traveling across the active area and to also prevent vapor from ascending through the downcomer. In one design known as a single pass arrangement, the inlet area that receives liquid discharged from an overlying downcomer and the opening or inlet for the downcomer that removes the liquid from the tray are positioned at opposite ends of each tray. In a two-pass arrangement, a single downcomer is centrally positioned on one tray and two downcomers are positioned at opposite ends of the underlying tray. Multiple-pass downcomer arrangements are also utilized.
Vapor-liquid cross-flow trays which use a plurality of cylindrical cans have been developed for high flow capacity and high efficiency applications. The cylindrical cans extend upwardly from the deck surface and surround vapor openings formed in the deck surface. A swirling motion is imparted to vapor ascending within the can using swirl vanes positioned within the cans. Liquid flowing on the deck surface is introduced into the cans through slots or other openings positioned in the lower portion of the can wall. One or more downcomers are positioned on each tray to remove liquid from one tray and deliver it to an underlying tray.
The swirling vapor ascends through these cans and interacts with the liquid entering the cans from the deck surface to cause vigorous vapor-liquid interaction that leads to high separation efficiency. The swirling vapor also causes a large portion of the liquid to be splashed against the inner wall surface of each can, where it passes through vertical and horizontal slots in the can wall. After passing through these slots, the liquid descends to the deck surface and then travels along the deck surface to one or more downcomers for passage to an underlying tray. The vapor exits an open top or other openings provided in each can and then flows upwardly through the vapor openings and the cans provided on the deck surface of the overlying tray.
One problem with the high capacity trays discussed above is that the liquid on the deck surface is uniformly distributed to each of the cans. At low liquid flow rates, the quantity of liquid presented to each can is such that it becomes readily entrained in the vapor stream and forms a spray which diminishes the separation efficiency of the tray.
Another problem associated with the design of these high capacity trays is the tendency of the liquid to be recycled to each can rather than flowing in the desired plug flow pattern across the tray.
A need has thus developed for an improved tray design that overcomes the problems described above.